There has been wide use of a fiber-optic system that includes a double clad fiber (hereinafter, also referred to as an “amplification double clad fiber”) that amplifies signal light and a single clad fiber (hereinafter, also referred to as a “transmission single clad fiber”) that transmits the signal light amplified by the double clad fiber. Typical examples of such a fiber-optic system are a fiber amplifier and a fiber laser.
FIG. 13 is a block diagram illustrating a configuration of a typical conventional fiber amplifier 5. As illustrated in FIG. 13, the fiber amplifier 5 is a fiber-optic system including a plurality of optical fibers. In the following description, the optical fibers constituting the fiber amplifier 5 are divided into four segments. These four segments are referred to as a first optical fiber 51, a second optical fiber 52, a third optical fiber 53, and a fourth optical fiber 54, respectively. Moreover, a fusion splice point between the first optical fiber 51 and the second optical fiber 52 is referred to as P2 and a fusion splice point between the second optical fiber 52 and the third optical fiber 53 is referred to as P3. Further, a fusion splice point between the third optical fiber 53 and the fourth optical fiber 54 is referred to as P4.
The first optical fiber 51 is an optical fiber for taking in signal light and made of a single clad fiber. The first optical fiber 51 has an incident end P1 to which a signal light source (not illustrated) is connected. After signal light enters the first optical fiber 51 from the signal light source via the incident end P1 and then propagates through the first optical fiber 51, the signal light enters the second optical fiber 52 via the fusion splice point P2.
The second optical fiber 52 is an optical fiber for taking in excitation light. In the second optical fiber 52, a pump combiner 56 is inserted. To this pump combiner 56, at least one (six in an example illustrated in FIG. 13) excitation light source 55 is connected. In the second optical fiber 52, a segment portion from the fusion splice point P2 to the pump combiner 56 is made of a single clad fiber and a segment portion from the pump combiner 56 to the fusion splice point P3 is made of a double clad fiber. After the signal light having entered a core of the second optical fiber 52 from the first optical fiber 51 via the fusion splice point P2 propagates through the second optical fiber 52, the signal light enters the third optical fiber 53 via the fusion splice point P3. Meanwhile, after excitation light enters a first clad of the second optical fiber 52 from the excitation light source 55 via the pump combiner 56 and then propagates through the second optical fiber 52, the excitation light enters the third optical fiber 53 via the fusion splice point P3.
The third optical fiber 53 is an optical fiber for amplifying the signal light and made of a double clad fiber. In other words, the third optical fiber 53 is an amplification double clad fiber. The third optical fiber 53 has a cross-section structure including a core 53a to which an active element such as a rare earth element is added, a first clad 53b1 surrounding the core 53a, a second clad 53b2 surrounding the first clad 53b1, a jacket 53c surrounding the second clad 53b2 (see FIG. 14). After the excitation light having entered the third optical fiber 53 from the second optical fiber 52 via the fusion splice point P3 propagates through the first clad 53b1 of the third optical fiber 53, the excitation light enters the fourth optical fiber 54 via the fusion splice point P4. The excitation light causes a transition of a state of the active element added to the core 53a to a state of population inversion. Meanwhile, after the signal light having entered the third optical fiber 53 from the second optical fiber 52 via the fusion splice point P3 propagates through the core 53a of the third optical fiber 53, the signal light enters the fourth optical fiber 54 via the fusion splice point P4. The active element added to the core 53a of the third optical fiber 53 is made to cause stimulated emission by the signal light. Because the active element is kept in the state of population inversion by the excitation light, the number of photons emitted in the stimulated emission exceeds the number of photons absorbed by the active element. In other words, the signal light having entered the third optical fiber 53 is amplified during a process in which the signal light propagates through the core 53a of the third optical fiber 53.
The fourth optical fiber 54 is an optical fiber for transmitting the amplified signal light and made of a single clad fiber. In other words, the fourth optical fiber 54 is a transmission single clad fiber. After the signal light having entered the fourth optical fiber 54 from the third optical fiber 53 via the fusion splice point P4 propagates through a core 54a of the fourth optical fiber 54, the signal light is outputted from an output end P5 of the fourth optical fiber 54.
In a case where the fourth optical fiber 54 that is made of a transmission single clad fiber is fusion sliced to the third optical fiber 53 that is made of an amplification double clad fiber as described above, the following problem occurs at the fusion splice point P4.
That is, in a case where axial misalignment occurs at the fusion splice point P4 between the third optical fiber 53 and the fourth optical fiber 54, part of signal light La1, La2 amplified by the core 53a of the third optical fiber enters a clad 54b of the fourth optical fiber 54, as illustrated in FIG. 14. Further, residual excitation light Lb, which has not been absorbed by the active element but remains, also enters the clad 54b of the fourth optical fiber 54, as illustrated in FIG. 14. The signal light La and the residual excitation light Lb that have entered the clad 54b of the fourth optical fiber 54 cause the jacket 54c to heat up, during a process in which the signal light La and the residual excitation light Lb propagates through the clad 54b of the fourth optical fiber 54. Due to this heat, the jacket 54c is degraded. In the worst case, the fourth optical fiber 54 may be broken. Particularly in a recent high-output fiber amplifier, a power of the signal light La2 propagating the clad 54b of the fourth optical fiber 54 reaches a level of tens of watts. Therefore, this problem is serious in such a recent high-output fiber amplifier.
Note that respective core diameters of the amplification double clad fiber and the transmission single clad fiber are approximately 10 μm in general. Therefore, a slight axial misalignment causes the signal light to enter the clad of the transmission single clad fiber from the core of the amplification double clad fiber. Further, in many cases, a shape of the core of the amplification double clad fiber is arranged to be polygonal so as to prevent a skew while a shape of the core of the transmission single clad fiber is arranged to be circular. In such a case, due to difference in core shape at the fusion splice point, the signal light easily enters the clad of the transmission single clad fiber from the core of the amplification double clad fiber.
Patent Literatures 1 through 4 discloses techniques for solving the above problem, respectively.
Patent Literature 1 discloses a technique for converting, into heat, residual excitation light that has just entered a single clad fiber. According to the technique, such residual excitation light is converted into heat, by (i) covering a fusion splice point between a double clad fiber and the single clad fiber with a block made of a material with a high thermal conductivity and (ii) filling, with a transparent resin, a space between this block and each of the double clad fiber and the single clad fiber. The transparent resin employed here is a resin having a refractive index that is higher than that of a clad of the single clad fiber. Patent Literature 2 also discloses a technique according to which residual excitation light is converted into heat with use of a heat dissipation plate, by covering a fusion splice point between a double clad fiber and a single clad fiber with a resin having a high refractive index.
Moreover, Patent Literature 3 discloses a technique for removing residual excitation light that is propagating through a clad of a single clad fiber. According to the technique, such residual excitation light is removed, by providing a guide member to the single clad fiber that is fusion spliced to a double clad fiber. This guide member is a cylindrical member which has a higher refractive index than the clad of the single clad fiber and which tightly adheres to the single clad fiber.
Further, Patent Literature 4 discloses a technique for removing residual leaking light that has entered a clad of a single-mode polarization maintaining fiber from a double-clad polarization maintaining fiber. The residual leaking light is removed by a configuration where: the single-mode polarization maintaining fiber from which approximately 10 cm of a jacket is removed is wound into a coil having a diameter of 30 mm; and this wound single-mode polarization maintaining fiber is fixed to a metal plate.